Message threading refers to the task of identifying sequences or threads of messages related to a single logical conversation, event or action. It has been typically used in newsgroups, discussion forums, and most notably in email. In email, the thread extension has been supported for a number of years in IMAP, the Internet Message Access Protocol, and several algorithms have been offered to implement this extension. Messages within a thread are typically organized chronologically. Most mail clients support threading today either under IMAP or in a more proprietary manner and typically visualize threads to ease reading, searching, browsing, labeling, etc. of the user's inbox.
The most common type of email threading is dialog based and identified through pure syntactic analysis. This type of threading demands from the messages in the thread to have been part of a dialog between senders and recipients. The dialog is easily identified through syntactic analysis of the subject as prefixes such as “Re:” or “Fw:” added to the subject line, and of the senders and recipients fields of the messages. For example, one email threading algorithm even defines threading as the action of “grouping messages together in parent/child relationships based on which messages are replies to which others.” These subject, recipient, and sender fields are part of the header of a mail message as formally defined in the earliest mail transfer protocols such as SMTP. This type of threading requires a dialog to have happened and does not cover one directional communication where one or several senders address the same recipient(s) around the same topic and are conceptually part of the same conversation.